The column known from U.S. Pat. No. 4,831,266 describes an electron-optical column comprising an electron source and a combined magnetic/electrostatic objective lens round the optical axis. The magnetic part of the lens comprises two pole pieces at ground potential, a first pole piece close to the sample and a second pole piece more removed from the sample.
The electrostatic lens consists of a first electrode in the form of an aperture round an optical axis near the sample. This first electrode coincides with the first pole piece. A second electrode in the form of a flaring tube surrounds the optical axis between the first electrode and the electron source. The tube has a small diameter at the side of the sample, and a larger diameter at the side of the electron source. The second electrode is kept at a potential UKE with respect to the sample and the magnetic pole pieces. A third electrode in the form of a tube with a small diameter is placed round the optical axis between the second electrode and the source. The third electrode is kept at a potential URE with respect to the sample.
A detector comprising for example a scintillator disk with light guide or a semiconductor disk (such as a PIN diode) surrounds the third electrode in a detector plane perpendicular to the optical axis. The detector is kept at the potential of the second electrode, UKE, the potential of the second electrode.
When the primary beam hits a sample, secondary radiation including secondary electrons (SE's, defined as electrons having an energy of less than 50 eV, more specifically having an energy of 5 eV or less) and back-scattered electrons (BSEs, defined as having an energy above 50 eV up to the energy of the impinging electrons) are generated.
The SEs are kept close to the axis of the objective lens due to the combined effect of the electrostatic and the magnetic field breaking out of the lens. The known patent teaches that a cross-over is formed between the sample and the detector, as a result of which a diverging beam illuminates the detector. The detector thus detects a large fraction of the SEs and also some of the BSEs.
A disadvantage of the column disclosed in the known patent is the low efficiency with which BSEs are detected.
U.S. Pat. No. 4,926,054 discloses a lens similar to the lens disclosed in U.S. Pat. No. 4,831,266 showing an additional control electrode between the sample and the yoke of the magnetic lens, so that the position of the cross-over of the SEs can be tuned. By placing an aperture at the position of the cross-over, BSEs are blocked and an image of almost exclusively SEs can be formed.
A disadvantage of the column disclosed in U.S. Pat. No. 4,926,054 is that the detection efficiency of BSEs is low and that still another control electrode and associated power supply is needed, resulting in a still more complex lens.
Accordingly, there is a need to provide a solution for efficiently detecting back-scattered electrons.